Clean Boundaries

Boundary code converts uncertain external behavior into explicit internal contracts. Keep unsafe shapes at the edge and pass typed values inward.

B1: Validate At The Edge

External data is unknown until validated. Parse and normalize once, then use domain types internally.

// Bad - assertion lets invalid data into the system
const user = JSON.parse(responseText) as User;

// Good - boundary handles parsing, validation, and source context
const user = parseUserResponse(responseText);